(1) Field of the Invention
The present invention relates to display devices and methods of driving the same, and particularly to a display device using current-driven luminescence elements, and a method of driving the same.
(2) Description of the Related Art
Display devices using organic electroluminescence (EL) elements are well-known as display devices using current-driven luminescence elements. An organic EL display device using such self-luminous organic EL elements does not require backlights needed in a liquid crystal display device and is best suited for increasing device thinness. Furthermore, since viewing angle is not restricted, practical application as a next-generation display device is expected. Furthermore, the organic EL elements used in the organic EL display device are different from liquid crystal cells which are controlled according to the voltage applied thereto, in that the luminance of the respective luminescence elements is controlled according to the value of the current flowing thereto.
In the organic EL display device, the organic EL elements included in the pixels are normally arranged in rows and columns. In an organic EL display referred to as a passive-matrix organic EL display, an organic EL element is provided at each crosspoint between row electrodes (scanning lines) and column electrodes (data lines), and such organic EL elements are driven by applying a voltage equivalent to a data signal, between a selected row electrode and the column electrodes.
On the other hand, in an organic EL display device referred to as an active-matrix organic EL display device, a switching thin film transistor (TFT) is provided in each crosspoint between scanning lines and data lines, the gate of a drive element is connected to the switching TFT, the switching TFT is turned ON through a selected scanning line so as to input a data signal from a signal line to the drive element, and an organic EL element is driven by such drive element.
Unlike in the passive-matrix organic EL display device where, only during the period in which each of the row electrodes (scanning lines) is selected, does the organic EL element connected to the selected row electrode generate photons, in the active-matrix organic EL display device, it is possible to cause the organic EL element to generate photons until a subsequent scan (selection), and thus a reduction in display luminance is not incurred even when the duty ratio increases. Therefore, the active-matrix organic EL display device can be driven with low voltage and thus allows for reduced power consumption. However, in the active-matrix organic EL display device, due to variation in the characteristics of the drive transistors, the luminance of the organic EL elements are different among the respective pixels even when the same data signal is supplied, and thus there is the disadvantage of the occurrence of luminance unevenness.
In response to this problem, for example, Japanese Unexamined Patent Application Publication No. 2008-122633 (Patent Reference 1) discloses a method of compensating for the variation of characteristics for each pixel using a simple pixel circuit, as a method of compensating for the luminance unevenness caused by the variation in the characteristics of the drive transistors.
FIG. 9 is a block diagram showing the configuration of a conventional image display device disclosed in Patent Reference 1. An image display device 500 shown in the figure includes a pixel array unit 502 and a drive unit which drives the pixel array unit 502. The pixel array unit 502 includes scanning lines 701 to 70m disposed on a row basis, and signal lines 601 to 60n disposed on a column basis, pixels 501 each of which is disposed on a part at which both a scanning line and a signal line cross, and power supply lines 801 to 80m disposed on a row basis. Furthermore, the drive unit includes a signal selector 503, a scanning line drive unit 504, and a power supply line drive unit 505.
The scanning line drive unit 504 performs line-sequential scanning of the pixels 501 on a per row basis, by sequentially supplying control signals on a horizontal cycle (1 H) to each of the scanning lines 701 to 70m. The power supply line drive unit 505 supplies, to each of the power supply lines 801 to 80m, power source voltage that switches between a first voltage and a second voltage, in accordance with the line-sequential scanning. The signal selector 503 supplies, to the signal lines 601 to 60n that are in columns, a reference voltage and a luminance signal voltage which serves as an image signal, switching between the two voltages in accordance with the line-sequential scanning.
Here, two each of the respective signal lines 601 to 60n in columns are disposed per column; one of the signal lines supplies the reference voltage and the signal voltage to the pixels 501 in an odd row, and the other of the signal lines supplies the reference voltage and the signal voltage to the pixels 501 in an even row.
FIG. 10 is a circuit configuration diagram for a pixel included in the conventional image display device disclosed in Patent Reference 1. It should be noted that the figure shows the pixel 501 in the first row and the first column. The scanning line 701, the power supply line 801, and the signal lines 601 are provided to this pixel 501. It should be noted that one out of the two lines of the signal lines 601 is connected to this pixel 501. The pixel 501 includes a switching transistor 511, a drive transistor 512, a storing capacitor 513, and a luminescence element 514. The switching transistor 511 has a gate connected to the scanning line 701, one of a source and a drain connected to the signal line 601, and the other connected to the gate of the drive transistor 512. The drive transistor 512 has a source connected to the anode of the luminescence element 514 and a drain connected to the power supply line 801. The luminescence element 514 has a cathode connected to a grounding line 515. The storing capacitor 513 is connected to the source and gate of the drive transistor 512.
In the above-described configuration, the power supply line drive unit 505 switches the voltage of the power supply line 801, from a first voltage (high-voltage) to a second voltage (low-voltage), when the voltage of the signal line 601 is the reference voltage. Likewise, when the voltage of the signal line 601 is the reference voltage, the scanning line drive unit 504 sets the voltage of the scanning line 701 to an “H” level and causes the switching transistor 511 to be in a conductive state so as to apply the reference voltage to the gate of the drive transistor 512 and set the source of the drive transistor 512 to the second voltage. With the above-described operation, preparation for the correction of a threshold voltage Vt(TFT) of the drive transistor 512 is completed.
Next, in the correction period before the voltage of the signal line 601 switches from the reference voltage to the signal voltage, the power supply line drive unit 505 switches the voltage of the power supply line 801, from the second voltage to the first voltage, and causes a voltage equivalent to the threshold voltage Vt(TFT) of the drive transistor 512 to be stored in the storing capacitor 513.
Next, the power supply line drive unit 505 sets the voltage of the switching transistor 511 to the “H” level and causes the signal voltage to be held in the storing capacitor 513. Specifically, the signal voltage is added to the previously held voltage equivalent to the threshold voltage Vt(TFT) of the drive transistor 512, and stored into the storing capacitor 513. Then, the drive transistor 512 receives a supply of current from the power supply line 801 to which the first voltage is being applied, and supplies the luminescence element 514 with a drive current corresponding to the held voltage.
In the above-described operation, the period of time during which the reference voltage is applied to the respective signal lines is prolonged through the placement of two of the signal lines 601 in every column. This secures the correction period for storing the voltage equivalent to the threshold voltage Vt(TFT) of the drive transistor 512 in the storing capacitor 513.
FIG. 11 is an operation timing chart for the image display device disclosed in Patent Reference 1. The figure describes, sequentially from the top, the signal waveforms of: the scanning line 701 and the power supply line 801 of the first line; the scanning line 702 and the power supply line 802 of the second line; the scanning line 703 and the power supply line 803 of the third line; the signal line allocated to the pixel of an odd row; and the signal line allocated to the pixel of an even row. The scanning signal applied to the scanning lines sequentially shifts 1 line for every 1 horizontal period (1 H). The scanning signal applied to the scanning lines for one line includes two pulses. The time width of the first pulse is long at 1 H or more. The time width of the second pulse is narrow and is part of 1 H. The first pulse corresponds to the above-described threshold voltage correction period, and the second pulse corresponds to a signal voltage sampling period and a mobility correction period. Furthermore, the power source pulse supplied to the power supply lines also shifts 1 line for every 1 H cycle. In contrast, the signal voltage is applied once every 2 H to the respective signal lines, and thus it is possible to ensure that the period of time during which the reference voltage is applied is 1 H or more.
In this manner, in the conventional image display device disclosed in Patent Reference 1, even when there is a variation in the threshold voltage Vt(TFT) of the drive transistor 512 for each pixel, by ensuring a sufficient threshold voltage correction period, the variation is canceled on a pixel basis, and unevenness in the luminance of an image is inhibited.